Genetics. Genetic problems A monohybrid cross is a cross of individuals looking at a characteristic...
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Transcript of Genetics. Genetic problems A monohybrid cross is a cross of individuals looking at a characteristic...
Genetics
Genetic problems
• A monohybrid cross is a cross of individuals looking at a characteristic inherited at one gene locus
• A test cross is crossing an individual back to a homozygous recessive individual in order to determine whether it is a carrier
• A Punnett square is a tool used in genetics• Genotype refers to the alleles present in
an individual• Phenotype refers to the characteristic
shown by the individual
b
B
b
b
Bb
bb
Bb
bb
Autosomal inheritance
Both males and females have 2 alleles for the characteristic
Homozygous individuals have 2 alleles the same and produce gametes with only 1 type of allele
Heterozygous individuals have 2 different alleles and produce two types of gametes with each allele
At fertilisation gametes combine so the new individual has 2 of each allele – one from each parent
We can show the probabilities of allele combinations from different crosses by using a Punnett square
Autosomal dominant/recessive
Individuals with two dominant alleles show the dominant phenotype
Individuals with two recessive alleles show the recessive phenotype
Individuals with one of each allele show the dominant phenotype
BB
bb
Bb
Autosomal dominant recessive crosses
Crossing a homozygous dominant individual with a homozygous recessive individual leads to offspring who are all heterozygous and show the dominant trait
Crossing two heterozygous individuals leads to 1 homozygous dominant individual, showing the dominant trait : 2 heterozygous individuals, showing the dominant trait :1 homozygous recessive individual, showing the recessive trait
BB bb
Bb Bb
BB Bb
Bb bb
B b
B
b
Example – dominant recessive problem
A heterozygous black male mouse mates with a homozygous brown female mouse. Black fur is dominant over brown fur. What is the probability of having:
a) a homozygous black offspring? 0% b) a heterozygous black offspring? 50% c) a homozygous brown offspring? 50%
Bb bb
b b
B Bb Bb
b bb bb
Autosomal co-dominance
Individuals with two of the 1st allele show the first trait
Individuals with two of the 2nd allele show the second trait
Individuals with one of each allele show a mixture of both traits
SBSB
SWSW
SBSW
Autosomal co-dominant crosses
Crossing an individual homozygous for one allele with an individual homozygous with the second allele leads to offspring showing a mixture of the two traits
Crossing two heterozygous individuals leads to 1 homozygous individual showing the first trait : 2 heterozygous individuals showing the mixed trait :1 homozygous individual showing the second trait
SBSB SWSW
SBSW SBSW
SB SW
SB SBSB SBSW
SW SBSW SWSW
Example – co-dominance problem
Two heterozygous grey sheep are mated. Black wool is co-dominant to white wool. What is the probability of having:
a) a black offspring? 25% b) a grey offspring? 50% c) a white offspring? 25%
SBSW SBSW
SBSB SBSW
SBSW SWSW
SB SW
SB
SW
Sex linked inheritance
Males and females have different chromosomesMales can only show 2 phenotypes (ie males
can not be carriers)Females can show 3 phenotypes (if codominant)
or 2 phenotypes (if dominant recessive, with a carrier)
You need to show alleles on the X chromosome (Y chromosomes don’t carry an allele)
Example – sex linked recessive problemIn humans, red-green colour blindness is a relatively common condition that is
inherited as an X-linked recessive trait. a) A woman with normal vision whose father was red-green colour-blind marries
a man with normal vision.i) What proportion of her sons would you expect to be colour-blind? 0ii) What proportion of her daughters would you expect to be colour-blind? 0
b) If she married a man who was red-green colour-blind, i) what proportion of her sons would you expect to be colour-blind? ½ ii) what proportion of her daughters would you expect to be colour-blind? ½
Xh
Xh
Xh
XH
XH
XH
XH Xh XhY
Y
Y
XH Xh
XH XH
Xh Xh
XHY
XhY
XHY
Genetics problem 12. In humans, tongue rolling is dominant over not being able to tongue roll. If a
hybrid tongue rolling male mates with a non-tongue rolling female and they have four children, determine:a) the probability that the first child is a tongue roller,b) the most probable ratio of tongue rollers to non-tongue rollers in the children,c) the most probable genotypes of each child.
Tt x tt T t
t
t
Tt tt
Tt tt
a) 50%b) 1:1c) ½ Tt, ½ tt
Genetics problems 2a
6. a) In humans, the ABO blood groups are controlled by a single genes with three alleles, IA, IB and i. IA is codominant with IB, and both are dominant over i. List each phenotype, and name the genotypes possible for each.
IA IA = blood type AIA i = blood type AIA IB = blood type ABIB IB = blood type BIB i = blood type Bi i = blood type O
Genetics problems 2b
6. b) A man of blood group O marries a woman of blood group A. The woman’s father was of blood group O. What are the chances that their children will belong to blood group O?
IAi x ii = ½ IAi (blood type A), ½ ii (blood type O)
Answer = 50%
Genetics problems 2c6. c) Four babies were born in a hospital on a night in which
an electrical blackout occurred. In the confusion that followed, their identification bracelets were mixed up. Conveniently, the babies were of four different blood groups: O, A, B and AB. The four pairs of parents have the following blood groups: O and O; AB and O; A and B; B and B. Which baby belongs to which parents?
O x O parents can only have baby O, the only parents that can produce an AB baby are A x B,the B x B parents can only produce a B or an O baby – so
they have baby B,so baby A must belong to parents AB x O
Genetics problems 3a7. Haemophilia is a disease carried as a sex-linked recessive trait.a) A haemophiliac male marries a homozygous normal female. What is the
probability of this family having ai) haemophiliac daughter? 0%
ii) haemophiliac son? 0%iii) carrier daughter? 50%
iv) carrier son? 0% v) normal son? 50%
vi) normal daughter? 0%
XH XH x Xh YXH XH
Xh XH Xh XH Xh
XH Y XH YY
Genetics problems 3b7. Haemophilia is a disease carried as a sex-linked recessive trait.b) One of the daughters, from this marriage is a carrier of haemophilia.
What is the probability of her daughter being a carrier, if her husband is normal? 50% of her daughters will be carriers
c) A haemophiliac male decides to wed a non-haemophiliac female whose father had haemophilia. What are the chances that the first child will be a son with haemophilia? 25%
XH Xh x Xh YXH Xh x XH Y
b)c)
Y Y
XH
XH XHXh Xh
XhXH Xh XH Xh
XH XHXh Xh
XH Y Xh Y XH Y Xh Y
Genetics problems 48. a) In humans, Duchenne muscular dystrophy is an X-linked recessive disease which is
very rare in females. The males who have the disease usually die before they reach 18 years old. How are these two facts related?
Girls can only have a sex-linked recessive disease if they have 2 affected X chromosomes – ie they have inherited affected chromosomes from both the mother and the father. As males rarely survive to breeding age, this is not likely to occur.
b) A female has been diagnosed as a carrier of the dystrophic gene. What are the chances that her sons will be normal and healthy? 50%
c) A woman whose brother died of muscular dystrophy wants to marry a man whose family has no history of the disease.i) What is the probability that she is a carrier? 50%ii) What is the probability that her first child has the disease? 1/8 or 12.5%
b) XD Xd x XD Y c) XD Xd x XD Y or XD XD x XD Y
XD
Y Y Y
XD XDXD
XD
XD XD
XD XDXdXd
XD Xd
XD XDXD XD XD XD XD Xd
XD Y Xd Y XD Y XD YXD Y Xd Y